Introduction 1 6 7 In this study the accuracy of the surgeons peroperative estimation of the position of the cup is evaluated. Materials and methods All patients undergoing primary total hip arthroplasty were enrolled in the study. Patient characteristics as name, sex, age, operated side and the body mass index were recorded. All cooperating surgeons (three orthopaedic surgeons and nine residents) received a list on which the following data concerning the operation were recorded: fixation (cemented or uncemented), cup model, surgical approach, complications and the peroperative estimated anteversion and abduction of the acetabular component. 8 9 t P Results Two hundred cups in 194 patients, placed between June 2003 and May 2005, were included in the study. There were 55 males and 139 females. At time of operation the mean age of the patients was 72.4 years (34–92) with a mean body mass index of 27.5 (16.6–38.1). The cups were placed by 12 different surgeons. Eighty-five cups (42.5%) were placed by three orthopaedic surgeons and 115 (57.5%) by 9 of their residents, always under supervision of one of the surgeons. . P 1 Table 1 Mean estimated and measured anteversion and abduction  N Anteversion (degrees) Difference Mean inaccuracy of estimation Estimated Measured Ia  Orthopedic surgeon 85 14.5 14.7 −0.2 5.2 (SD 4.5)  Residents 115 14.7 16.9 −2.2 5.7 (SD 5.0)  Total 200 14.6 (SD 5.9) 16.0 (SD 8.1) −1.4 5.5  Significance  – – – –  N Abduction Difference Mean inaccuracy of estimation Estimated Measured Ib  Orthopedic surgeon 85 47.2 48.6 −1.4 4.09 (SD 3.9)  Residents 115 45.6 50.5 −4.9 6.28 (SD 4.6)  Total 200 46.3 (SD 4.3) 49.7 (SD 6.7) −3.4 5.4  Significance  0.001 0.043 0.000 0.001 1 Fig. 1 a b N  2 3 X Table 2 Results of the chance for cup placement within the limits according to Lewinnek (5–25° anteversion and 30–50° abduction) for the orthopedic surgeons, their residents and the entire group. Intended cup placement was virtually set on 15° anteversion and 40° abduction  Anteversion (%) Abduction (%) Placement according to Lewinnek (%) Orthopedic surgeons 85.7 93.7 80.3 Residents 80.5 78.9 63.5 Group 82.7 85.2 70.5 Table 3 X  X X X X X (a) Group  Abduction 100 85.2 46.8 26 16.4  Anteversion 99.9 82.7 46 26.7 17.5  Combined 99.9 70.5 21.5 6.9 2.9 (b) Surgeon  Abduction 100 93.7 59.3 34 21.2  Anteversion 99.9 85.7 48.3 27.5 17.6  Combined 99.9 80.3 28.6 9.4 3.7 (c) Residents  Abduction 99.8 78.9 39 20.7 12.7  Anteversion 99.8 80.5 44.5 26.2 17.4  Combined 99.6 63.5 17.4 5.4 2.2 A multivariate regression analysis was applied in order to identify any factors that might be responsible for the inaccuracy of the estimation. A significant result was found for abduction and anteversion, concerning age of the patient and if the patient was operated by an orthopaedic surgeon or resident. Other factors like body mass index, sex, operated side, (un)cemented fixation of the acetabular component, model of the cup and the surgical approach did not reveal any significant differences. Discussion Malposition of the acetabular cup is probably the most important factor for dislocation of a total hip prosthesis. Therefore it is essential that the surgeon has maximum control over the position of the socket during the operation. Free hand positioning with the patient in a standardized position and a cup positioner with the patient and floor as reference is the routine method. Specific mechanical alignment guides were designed to add precision, as do navigation systems in the concept of CT-guided computer-assisted surgery. Surprisingly, there are very limited data on the precision rate of free hand positioning of the acetabular shell. The aim of our prospective study was to determine the accuracy of the free hand technique. Only then the presently introduced systems can be tested against this standard. The term “free hand” is not used uniformly in the literature. Either it is referred to pure manual positioning or with the aid of the acetabular cup impactor-positioner provided with the implant. In the present study, the latter definition was applied. 10 11 12 Without doubt it is essential to actually place what is aimed for. Our present study shows that the 70.5% positioned within the safe zone is remarkably good for free hand placement compared with literature. But when the target is reset from the wide safe zone of Lewinnek to for instance within 5° or even 1° of error from the judged position for both abduction and anteversion, only 21.5 and 2.9% of our sockets are placed within this narrow definition of safe zone. We believe that the power of our study comes from the distinctive set up where the recorded data are a reflection of the surgeon’s perception of cup placement. To our opinion this is superior then to allow for a wide range or set a predefined target. At the actual point of cup placement a surgeon is also committed to or governed by the patient’s anatomy, which will influence the actual position. For instance when less anteversion is accepted in the socket; the stem will be given some additional anteversion. 11 13 Considering the estimation of abduction, significant results were found in favour of the orthopaedic surgeons compared to their residents. A learning curve as a result of experience is probably responsible for this difference. As to surgical approach, acetabular components placed posterolaterally were measured to have a significantly higher-degree of anteversion and abduction. This is obviously related with the intention to minimize the change of a dislocation for both approaches. Although both approaches take a different anatomical route to the acetabulum, visualization and presentation of the acetabulum do not seem to be important factors. The conclusion is that an anterolateral or posterolateral approach is not a distinguishing factor in the accuracy of cup placement in our series. 14 14 From this prospective study it is concluded that an accuracy of 70.5% for placement of the socket with an acetabular shell imactor-positioner within Lewinnek’s safe zone and of only 21.5% within an error of 5° for both abduction and anteversion is unacceptable. The strength of our research is that literature is extremely scarce on studies as our prospective study on free hand positioning without special aiming devices. The results of the present study appear convincing but limitations are the high-number of participating surgeons, different surgical approaches (anterolateral and posterolateral) and two methods of cup fixation (uncemented and cemented). Multivariate analysis on these factors revealed only differences between the achievements of surgeons and residents. The consequence of our conclusion that free hand cup positioning is an unreliable method is that attention now should be focussed on the results of a randomized trial comparing the different techniques of cup positioning.